1. Field of the Invention
The present invention relates to the field of transfer function generators. More specifically, the present invention relates to a low voltage, high frequency cubic function generator.
2. Description of the Related Art
Function generators are circuits having an output-input characteristic which can be set to approximate a given curve. Circuits that generate an output having a cubic relationship to the input are known as cubic function generators.
Function generators are used for a variety of purposes. For example, in wireless communication systems, a function generator may be used to tune a power amplifier to obtain the desired output from the power amplifier. The function generator provides a tuning signal to the power amplifier to offset the natural behavior of the amplifier.
In many systems today, especially in the digital environment, function generators are required to perform at high speeds. For example, many circuits operate at frequencies in the gigahertz range. In addition to the speed requirements, many digital systems are low voltage, requiring a function generator capable of operating at approximately 2 volts. If the function generator only needs to produce a quadratic function, the voltage criteria is easily met. However, existing circuits do not meet these specifications when a cubic function is required.
What is needed is a compact, cubic function generator capable of operating at high frequencies and low voltages. Specifically, a circuit is required that generates a cubic function while operating at approximately 2 volts and at frequencies up to and including the gigahertz range.
The present invention generates a cubic transfer function for low voltage, high frequency applications. The cubic function generator of the present invention may be used, for example, in a control loop to linearize the output of a power amplifier in high speed wireless communication circuits.
The present invention generates a cubic transfer function while maintaining a voltage drop across only two active devices. This allows the present invention to operate with low voltage applications, specifically applications requiring a voltage drop of approximately 2 volts. Also, the circuit of the present invention may be formed on a semiconductor wafer, thereby enabling the circuit to operate at high frequencies.
One embodiment of the present invention comprises a cubic function generator comprising a first current source having a first terminal and a second terminal and a first transistor having a base, an emitter, and a collector, wherein the second terminal of the first current source is coupled to the base of the first transistor and the second terminal of the first current source is coupled to the collector of the first transistor. A first diode and a second diode are included, wherein the anode of the first diode is coupled to the second terminal of the first current source, the cathode of the first diode is coupled to the anode of the second diode, and the cathode of the second diode is coupled to ground. A second transistor has a base, an emitter, and a collector, wherein the emitter of the first transistor is coupled to the base of the second transistor, the emitter of the second transistor being coupled to ground. A third transistor has a base, an emitter, and a collector, wherein the emitter of the first transistor is coupled to the collector of the third transistor, the emitter of the third transistor being coupled to ground. A third diode and a fourth diode are included, wherein the cathode of the third diode and the anode of the fourth diode are coupled to the base of the third transistor while the cathode of the fourth diode is coupled to ground. A second current source has a first terminal and a second terminal, the second terminal of the second current source being coupled to the anode of the third diode and the first terminal of the second current source being coupled to the collector of the first transistor. A fourth transistor has a base, an emitter, and a collector, wherein the base of the first transistor is coupled to the anode of the third diode and the second terminal of the second current source is coupled to the collector of the fourth transistor. A fifth transistor has a base, an emitter, and a collector, wherein the emitter of the fourth transistor and the collector of the second transistor are coupled to the base of the fifth transistor, while the emitter of the fifth transistor is coupled to ground.
One embodiment of the invention comprises a first current source (I3) and a second current source (Ic). The second current source is dependent from the first current source so as to create an output (Iout) that is a cubic function of the second current source in relation to the first current source so that:       I    out    =            I      3      3              I      c      2      
wherein the maximum voltage of the cubic function generator is 2* (Vbe+Vce(sat)).